1. Field of the Invention
The present invention relates to a magnetic resonance imaging method (hereinbelow referred to as MRI method) in a magnetic resonance imaging (MRI) device and a device therefor in which nuclear magnetic resonance (hereinbelow referred to as NMR) signals from such as hydrogens and phosphoruses in an object to be inspected are measured, and such as a nuclear density distribution and a relaxation time distribution therein are imaged, and, in particular, relates to an MRI method and a device therefor which eliminate artifacts due to an object motion, while keeping a high temporal resolution for animating images.
2. Conventional Art
In such fields as multi-shot echo planar imaging (multi-shot EPI) and three dimensional (3D) EPI, a navigation echo method for reducing artifacts due to object motions, which are caused in images because of motions of an object to be inspected between shots, has been known, for example, as described in Seong-Gi Kim et al. "Fast Interleaved Echo-Planar Imaging with Navigator: High Resolution Anatomic and Functional Images at 4 Tesla" (Magnetic Resonance in Medicine, 35: 895-902, June 1996).
A navigator echo is, for example, generated by applying a gradient magnetic field pulse for generating the navigator echo between a RF pulse 201 and a series of echo signal train obtaining routine 211 in a multi-shot EPI sequence as illustrated in FIG. 4. Such sequence is most particularly illustrated in FIG. 6. Wherein one navigator echo 3021 and 3022 is introduced for every RF pulse 2011 and 2012 irradiation. While assuming that no positional fluctuation of an object to be inspected occurs within a predetermined repetition time 209 or 304, motion of the object to be inspected is estimated based on a change between one navigator echo obtained in one repetition time (TR) and another navigator echo obtained in a subsequent repetition time (TR).
As illustrated, for example, in FIG. 7, when repeating a repetitive unit measurement 304 each including one navigator echo for obtaining one piece of image, four times in a unit 305 a navigator echo in a first measurement (illustrated in the drawing in gray) corresponding to shot number 1 is used as a reference and a necessary correction with regard to an object motion is carried out for the data of corresponding actual measurements (207 in FIG. 4) based on the changes of the navigator echoes in the following three time unit measurements 304 (shot numbers 2-4) to thereby obtain one piece of image 6011.
On the other hand, in order to obtain a magnetic resonance (hereinbelow called as MR) animating image, an MR fluoroscopy is known as one of applications of multi-shot EPI. Such MR fluoroscopy is illustrated in FIG. 5 in which from the beginning of measurement to the end thereof repetitive unit measurements 209 are time to time continued to obtain echo signal trains, and a predetermined number of new echo signal trains which are necessary for reconstructing one piece of image are used while counting back from the latest measurement, For example, in FIG. 5 echo signals obtained in four times of unit measurements are used, to reconstruct images for respective moments.
Even in such MR fluoroscopy, artifacts due to an object motion between shots are problematic. However, no MR fluoroscopy, to which a navigation echo method is applied, has been proposed.
In order to correct such an object motion it is conceivable to combine the above mentioned MR fluoroscopy and the navigation echo method. However, if both methods are simply combined, when reconstructing pieces of images 5011.about.5017 while successively shifting data for respective image reconstruction by one unit measurement 304, a navigator echo in a predetermined repetitive unit measurement having a fixed shot number among four repetitive unit measurements having respective shot numbers is used as a reference of phase correction for respective images, example, a navigator echo in the first repetitive unit measurement (as illustrated in gray in the drawing) is used as illustrated in FIG. 8.
In this instance, pieces of image 5011.about.5017 are renewed for every repetition time TR of the unit measurement 304. However, the navigator echo used as a reference is renewed for every four unit measurements counting from the first unit measurement 304 (the renewal timings A, B and C are illustrated in gray in the drawing), Therefore, a reference interval for the object motion correction making use of such navigator echo is prolonged to four times of the repetition time (4TR), and thus, notwithstanding a high apparent temporal resolution corresponding to the image renewal interval TR, a sudden change in an animating image is caused for every fourth image and the realtime characteristic thereof is deteriorated.